Because Nidhi has past history with Mallika and hence reason to question Ashutosh on extending help to her..
However, Ashutosh and Amar do not share past history and hence Ashutosh would be completely unjustified in questioning her acts...
After standing up for his principles and his commitment to humanity, if Ashutosh questioned Nidhi's behaviour now, it would render him incredibly hollow and worse still, a hypocrite...
If the purpose of this track is to show how all of us are hypocrites at the end of the day, then I suppose the writer is perfectly justified in taking this route...
I don't know if I can root for a hero who is a hypocrite!
These are just my personal views. You are welcome to disagree!